The invention relates to a catalytic converter and to a method for producing a catalytic converter.
Laid-open Specification JP-A-08 134 682 describes an electroplating method for coating a metallic substrate with a smooth noble metal layer in which a substrate containing iron is provided with a platinum coating.
Patent Specification DE 197 32 170 C2 discloses a method for locally and selectively electrochemically coating a ceramic SiC substrate with a platinum coating. A large surface area is achieved by matching the platinum coating to the rough ceramic surface. The coated substrate is then treated at a raised temperature of more than 400xc2x0 C.
EP 0 106 197 A2 discloses a method for depositing platinum on a carbon substrate or a semconductor substrate, in which the substrate has a first initial pulse applied to it, followed by a second, longer voltage pulse. The first initial pulse reaches voltage values at which platinum seeds are formed on the substrate surface, while the second pulse leads to the growth of the seeds that have been formed.
An object of the invention is to specify a catalytic converter and a method for producing a catalytic converter which allows the deposition of a catalytically active material with a large surface area and good adhesion strength on a substrate.
The object may be achieved by a catalytic converter and of a method for producing a catalytic converter according to the precharacterizing clauses of the independent patent claims. Further advantages and refinements of the invention are described in the other patent claims and in the description.
According to the invention, a catalytic converter is produced by depositing a layer of catalytically active material on a planar, electrically conductive substrate. In the process, a high overvoltage, which is in a voltage range within which a large number of seeds of the catalytic material are formed on the substrate in the chosen system, is set between the substrate and the opposing electrode for a predetermined first time period. The overvoltage is then reduced, for a predetermined second time period, to a value at which the seeds deposited in the first time period show a growth in size on the substrate.
The deposited layer preferably has metal clusters, which are firmly and directly connected to the substrate. The advantage is that the catalytically active material makes good thermal contact with the substrate, and there is also a very large active surface area without any additional supporting bodies. Furthermore, the adhesion is very good without any need to use costly adhesion promoter layers between the catalytic material and the substrate.
A voltage in the high seed formation range is preferably set repeatedly, followed by a voltage in the controlled seed growth range, particularly preferably periodically and successively.
In further preferred developments, the seed formation range and the seed growth range are set with a different time interval with respect to one another and/or with different voltage levels and/or different oscillations.
It is particularly preferable for at least two AC voltages to be superimposed, and for the superimposed sum voltages to be applied between the substrate to be coated and the opposing electrode. In the process, it may be advantageous to superimpose a DC voltage on the AC voltage as well.
In one advantageous embodiment, the surface to be coated of the substrate is roughened before the coating process, thus forming a mean surface roughness which corresponds approximately to the thickness of the layer.
Metal and/or ceramic and/or carbon is used as the preferred substrate.
Using the method according to the invention, it is possible in a simple and well-controllable manner to produce the deposition of well-adhering catalytically active layers with a very large active surface area, and with a consumption of catalytic material being low. The numbers of seeds and the clustered size can be set reproducibly, and can be optimized for different chemical systems by adaptation of the electrical parameters in a manner which can be controlled well and is easy to derive.